DE69807353T2 - IMPROVEMENTS IN THE GLASS COATING - Google Patents

Description

The present invention relates to the coating of glass and, in particular, to the application of a silica coating to flat glass at temperatures below swimming pool temperatures.

It has been known to apply silica coatings to glass at swimming pool temperatures (about 600°C and above) using a gaseous mixture of silane, ethylene and usually an oxygen-containing gas such as CO2 (see for example 6B I 573 154, EP 0 275 662 B and EP 0 348 185 B) and more recently it has been proposed in EP 0 611 733 A2 to deposit mixed coating layers containing both tin oxide and silica using, inter alia, alkoxysilane compounds as a source of the silica with an accelerator to increase the growth rate of the coating. EP 0 611 733 A2 discloses accelerators for coating systems operating above 1000ºF (538ºC, typically at swimming pool temperatures) and suggests the use of a wide range of accelerators including Lewis acids, Lewis bases, water, certain compounds of nitrogen, phosphorus, boron and sulfur with specified structural formula, certain aluminum compounds with specified structural formula and ozone. The only accelerators used in the examples, all of which are carried out at a glass transition temperature of 1200ºF (650ºC), are trialkyl phosphites.

Unfortunately, the processes known for applying silicon oxide (particularly silicon oxides containing a high proportion of oxygen resulting in a refractive index of 1.5 or less) essentially at atmospheric pressure have one or more of the following disadvantages when used to apply coatings to hot glass at temperatures at or below swimming pool temperatures, especially temperatures below 550ºC:

(a) Low coating growth rate,

(b) expensive reactants,

(c) tendency to deposit reactants in gas phase, leading to unacceptable particle formation,

(d) Risk of explosion when handling the reactant gas mixture used.

We have now found that one or more of these disadvantages can be eliminated or at least greatly mitigated by using a mixture of a silicon source and ozone-enriched oxygen to apply the coatings.

According to one aspect of the present invention, there is provided a method of applying a silicon oxide coating to hot glass at a temperature below 500°C, which comprises contacting the hot glass with a gaseous mixture of a silicon source and ozone-enriched oxygen. characterized in that the molar ratio of ozone to silicon source in the gaseous mixture is in the range from 0.01:1 to 0.4:1.

The silica coating may be stoichiometric or non-stoichiometric and may comprise components such as nitrogen, carbon and organic agents. The process is usually carried out by contacting the hot glass in the form of a hot glass ribbon with the gaseous mixture during the float glass manufacturing process downstream of the pool in the tunnel annealing furnace or in the space between the pool and the tunnel annealing furnace.

The hot glass will typically have a temperature in the range from 200 to 600ºC, preferably from 200 to 575ºC, and more preferably from 225ºC to 500ºC.

The application process of the present invention is most beneficial at temperatures of at least 350°C and preferably at least 375°C to ensure a high application rate. Since it is applicable at relatively low temperatures, it is particularly suitable for use when a coating of silica is to be applied to a float glass outside the swimming pool, for example in the tunnel annealing furnace or in the space between the swimming pool and the tunnel annealing furnace.

It is also particularly applicable when the temperature of the substrate is below about 525°C, when the deposition rate achieved with alternative silane-oxygen systems begins to drop significantly. Thus, in a particularly preferred aspect of the invention, the glass is contacted with the silicon source/oxygen/ozone mixture when the glass has a temperature in the range of 375°C to 525°C.

The hot glass is normally brought into contact with the gaseous mixture at essentially atmospheric pressure.

A wide range of silicon compounds have been used and proposed for use as a silicon source in vapor deposition processes, including in particular silanes and siloxanes. The suitability of a particular silicon compound for use in the processes of the present invention can be determined by routine experimentation. Those compounds which do not form coatings rapidly at the relatively low temperatures used in the processes of the present invention are less preferred for use in float glass manufacturing processes, although they may be useful in "off-line" deposition processes where the deposition time can be extended. Examples of suitable silanes include silane (SiH4), disilane, alkylsilanes (for example tri- or tetramethylsilane, hexamethyldisilane and other alkylsilanes having straight-chain or branched, substituted or unsubstituted alkyl groups having 1 to 12 carbon atoms), especially dialkylsilanes, preferably dimethylsilane;

Alkoxysilanes (for example methyltrimethoxysilane, dimethyldimethoxysilane and other alkylalkoxysilanes having substituted or unsubstituted, straight-chain or branched-chain alkyl groups with 1 to 12 carbon atoms), especially tetra(alkoxy)silanes such as tetraethoxysilane (TEOS); di(alkoxy)silanes, for example diacetoxy-di-tert-butoxysilane and oligomeric silanes, especially oligomeric alkoxysilanes, for example ethyl silicate 40. Examples of suitable siloxanes include hexa(alkyl)disiloxanes such as hexamethyldisiloxane and cyclic silanes, especially tetra(alkyl)cyclotetrasiloxanes such as tetramethylcyclotetrasiloxane, and the octa(alkyl)cyclotetrasiloxanes such as octamethylcyclotetrasiloxane (OMCTS). A silicon halide, for example silicon tetrachloride, can be used as the silicon source. The silicon source can comprise a mixture of two or more silicon compounds.

The preferred alkoxysilane, tetraethoxysilane, undergoes pyrolysis with oxygen to produce silane at decomposition rates which are practically only useful for coating glass in a line at temperatures above 650ºC. The deposition rate can be increased by using a plasma enhanced or low pressure CVD technique, but neither is suitable for commercial use on a continuous glass ribbon. Surprisingly, enrichment of the oxygen with only a small amount of ozone enables silica coatings with a high oxygen to silicon ratio (about 2, which gives a coating with a refractive index of 1.5 or less) to be formed at temperatures which are at least 375ºC low, at a rate which is suitable for practical use in on-line coating of glass, applied to hot glass substantially at atmospheric pressure, that is, without the need to use a vacuum or low pressure process such as sputtering which would not be practical for commercial on-line application.

A gas stream comprising ozone-enriched oxygen suitable for use in the processes of the invention may conveniently be prepared by passing an oxygen stream through an ozone generator. Ozone generators of this type are available as commercial products and are capable of producing a range of ozone concentrations in the oxygen stream. In a preferred embodiment of the present invention, an ozone-enriched oxygen stream produced by a conventional ozone generator is mixed with a second gas stream comprising at least one silicon source in a carrier gas to form a gaseous mixture which is contacted with the hot glass. The precise ozone concentration in the gaseous mixture is not normally critical and will depend on the concentration of silicon compound used, and an increase in the ozone concentration in the oxygen stream above 1% by weight based on the total weight of oxygen and ozone in the oxygen stream may result in little or no increase in the deposition rate. In fact, we have found that an increase in the ozone concentration in the gaseous mixture beyond a certain level may result in a reduced deposition rate, presumably due to competing reactions; in particular, competing reactions in the gas phase which result in powder formation become more significant at high ozone concentrations. Thus, for particular applied conditions, the ozone concentration in the gaseous mixture is adjusted to optimize the deposition rate. Typically this will involve using an ozone concentration (in the oxygen stream prior to mixing with the silicon source in the carrier gas) such that the oxygen is enriched with 0.5 to 10 wt% ozone, specifically that the oxygen is enriched with 0.5 to 5 wt% ozone, and that the molar ratio of ozone to silicon source compound in the gaseous mixture is not higher than 1:1, and preferably in the range 0.005:1 to 0.4:1, particularly 0.05 to 0.2:1. A low ozone concentration in the gaseous mixture is advantageous because, although higher ozone concentrations (up to a certain level) result in denser films, they also tend to promote gas phase reactions under certain application conditions. In addition, ozone is toxic and a lower ozone concentration is safer.

The ozone/oxygen gas is preferably mixed with the carrier gas and the vaporized silicon source before the gases are brought into contact with the hot glass surface. This mixing is preferably carried out immediately before the gases come into contact with the glass in order to reduce side reactions prior to deposition.

The present invention is illustrated, but not limited, by the following examples. In the examples, gas volumes are given at room temperature and pressure (i.e., at about 20°C and atmospheric pressure) unless otherwise indicated. Precursor liquid volumes at room temperature are liquid volumes prior to vaporization.

Ozone was produced from flowing oxygen gas using commercial ozone generators (Peak Scientific OZO6, Astex AX8100 or Wedeco Ltd. SWO30). Unless otherwise stated, ozone concentrations are the weight of ozone (in g/m3) or the percent weight of ozone in oxygen in the oxygen stream produced by the generator. The oxygen, enriched with ozone at this concentration, was then mixed with carrier gas containing the silicon source to form the gaseous mixture.

Examples 1 and 2

In these preferred examples, a silica coating was applied to an edge of a moving belt of float glass in the tunnel lehr during the production process using a laminar flow application method as described and illustrated in GB Patent 1 507 996 B, in which a coating chamber opening was applied to the glass from above with a length of 150 mm in the direction of belt movement and with a width of 100 mm was used. The glass was clear float glass with a belt thickness of 1.1 mm, moving at a belt speed of 348 meters per hour. The coating gas was applied when the glass temperature was in the range of 485 to 510ºC. In Example 1, the silicon source used was tetraethoxysilane (TEOS). Nitrogen carrier gas was passed through a TEOS wash bottle. The amount of TEOS in the nitrogen gas stream was regulated by changing the temperature of the TEOS in the wash bottle with a hot plate. The vapor pressure of TEOS changes with temperature in a known manner. The partial pressure of TEOS can therefore be regulated by changing the temperature of the TEOS in the wash bottle. The typical flow rate of the nitrogen carrier gas was 1 liter/minute. The amount of TEOS used was calculated using the change in liquid volume in the wash bottle. TEOS was added at 1.25 to 1.5 milliliters/minute (measured as liquid volume). The resulting gaseous TEOS stream in nitrogen carrier gas at 110ºC was mixed with an oxygen stream (typical flow rate 1 liter/minute) containing about 1 wt% ozone in the coating chamber. The molar ratio of ozone to TEOS was about 0.05:1.

The resulting coating, which was found to be silica with a refractive index of 1.46 and had good smoothness and uniformity (when examined using a scanning electron microscope), had a Thickness of 450 Å, which corresponds to a deposition rate of about 300 Å per second.

Example 2 was carried out under the same conditions as Example 1, except that octamethylcyclotetrasiloxane (OMCTS) was used as the silicon source, but using the same feed rate as for TEOS in Example 1, the molar ratio of ozone to OMCTS was about 0.06:1. The coating thickness in this case was slightly smaller than that observed in Example 1, at 350 to 400 Å, but the coating was again made of silica with a refractive index of 1.46, had good smoothness and uniformity when observed with the scanning electron microscope and atomic force microscope (compare SEM and AFM).

Examples 3 to 7

In each of these examples, a 4 mm thick sample of clear float glass with an area of about 50 square centimeters was heated on a graphite heater in a silica tube reactor and contacted with a gaseous mixture of an alkoxysilane and ozone-enriched oxygen flowing through the silica tube over the glass sample. The carrier gas containing the silicon source and the ozone-enriched oxygen stream were mixed as close to the hot glass as possible to avoid pre-reaction and powder formation.

The gaseous mixtures were prepared by injecting liquid tetraethoxysilane (TEOS) into a vaporizer where it vaporized in a nitrogen stream and was carried through the heated tube by the nitrogen, mixing it with an ozone-enriched oxygen stream immediately before it was introduced into the reactor. The ozone-enriched oxygen was generated using an ozone generator (Peak OZO6 for ozone concentrations below 2.5 wt% and Astex AX8100 for ozone concentrations above 2.5 wt%). The molar ratio of ozone to TEOS in the gaseous mixture was approximately 0.07:1.

In each example, deposition was continued for 15 to 20 seconds. After deposition, the flow of reactant gases was stopped and the sample was allowed to cool under a stream of nitrogen. Typically, about 60 minutes were required for a sample to cool to room temperature. After cooling, the samples were removed and the thickness of the coatings was measured after an appropriate etching treatment with 2% hydrofluoric acid using a stylus technique (a Dectak II profilometer) and/or a scanning electron microscope. The measured thickness of the coating and the deposition time were used to calculate the deposition rate. For example, the substrate temperature, the composition of the gas mixture and the deposition rate are given in Table 1. The composition of the coatings was determined for representative examples by creating an Auger profile, with the results indicating that the coatings consisted of silica with a slight excess of oxygen (25 to 30% Si, 65 to 70% O). However, the Auger profile is subject to inaccuracies and may be affected by interactions with the glass surface. The silicon:oxygen ratio is consistent with silica coatings, with the apparent excess of oxygen probably indicating that the coatings are porous. The refractive index of the coatings (general appearance of the coated glass) was used to monitor the deposition process.

As indicated in Table 1, the deposition rate for the silica coatings increased rapidly between deposition temperatures of 375ºC and 475ºC. Above 475ºC, the rate of increase in deposition rate is slower. A surprising result is that the deposition rate is reasonably high even at 375ºC, as shown in Example 3. This is important for the formation of silica coatings during glass manufacture.

Comparative Examples A, B and C were run under similar conditions to Examples 5 to 7 but in the absence of ozone; the conditions used and the deposition rates achieved are as shown in Table 2. When the examples are compared to the comparative examples, the improved deposition rate achieved using ozone in the steam is evident. In Example 5, compared to Comparative Example A when applied at 475ºC the deposition rate of the coatings from the ozone-containing vapor is 100 Å/s compared to 20 Å/s; a similar improvement is seen in Examples 6 and 7 compared to Comparative Examples B and C.

In each case, a silica coating identified by Auger profiling or refractive index was applied at the deposition rate indicated in Table 3.

Examples 8 to 16

These examples were run under the same conditions as Example 7 and Comparative Example C, except that the ozone concentration used varied from 0.29 wt% (Example 8) to 10.88 wt% (Example 16). The molar ratio of ozone to TEOS was between about 0.01:10 (Example 8) and about 0.3:1 (Example 16).

In each case, a silica coating, identified by Auger profile or refractive index, was applied at the deposition rate indicated in Table 3.

It is immediately apparent from Table 3 that ozone in the gas stream leads to an improvement in the deposition rate even when the ozone concentration is very low, as in Example 9. As the ozone concentration is further increased, the deposition rate continues to increase, but at a slower rate, as shown in Examples 10, 11 and 12. This effect is believed to be due to the fact that at this ozone concentration most of the TEOS has been oxidized, i.e. the TEOS concentration is the rate limiting value. The deposition rate remains at about the same level until the ozone concentration is 8 wt.% as in Example 15. Above 8 wt.%, Example 16, the deposition rate drops significantly, presumably because a pre-reaction takes place in the gas phase.

It will be seen that slightly higher deposition rates will occur at higher temperatures and it is also likely that the ozone concentration required to reach the saturation point (i.e., the concentration at which the deposition rate is substantially independent of the ozone concentration) and the ozone concentration above which the deposition rate drops significantly will be higher. The higher the ozone concentration, the better the surface quality and film density (roughly estimated from the change in etch rate) of the deposited coatings. If no ozone is supplied, as in the comparative examples, the optical quality and surface quality of the coating, even at 575ºC, are not good enough for commercially viable products. TABLE 1

* by fluid injection

** Varying the nitrogen flow rate over this range appears to have little effect on the deposition rate

*** approximate flow rate TABLE 2

Examples 17 to 21

These examples were run under the same conditions as Example 7, except for the TEOS flow rate. The conditions for these examples are given in Table 4. The molar ratio of ozone to TEOS varied between about 0.4:1 (Example 17) and about 0.05:1 (Example 21). The TEOS flow rate was regulated by an automatic syringe that injected controllable amounts of TEOS into the vaporizer. As the liquid TEOS feed rate is increased from 0.4 ml/min to 3.4 ml/min, the deposition rate also increases, as shown in Examples 17 through 20. When the TEOS feed rate in Example 20 is 3.4 ml/min, the deposition appears to have reached saturation since further increasing the TEOS feed rate, as shown in Example 21, does not increase the deposition rate. It is believed that further increasing the TEOS concentration after saturation results in partial oxidation of TEOS and a reduction in material efficiency. These examples and Examples 8 through 16 demonstrate that increasing either the ozone or TEOS concentration can increase the deposition rate until a saturation point is reached. The choice of conditions was judged in practice as a balance between deposition rate and other related effects including the cost of the starting materials.

Examples 22 to 29

These examples were carried out using the same apparatus and similar conditions as Examples 1 and 2. The gaseous mixture was applied when the glass transition temperature was in the range of 450 to 540°C.

The silicon source in Examples 22 to 29 was OMCTS.

Table 5 describes the OMCTS volume used per minute, the flow rates for nitrogen and oxygen, and the ozone concentration in the oxygen stream.

Examples 30 to 34

These examples were performed using the same apparatus and under similar conditions as Examples 3 to 7. The silicon source was OMCTS.

The oxygen flow rate was 2 l/min. enriched with 5.7 g/m³ ozone (measured with an ozone meter H1 obtained from IN USA Inc.). The liquid precursor was injected into nitrogen carrier gas (flow rate 2 l/min) at 0.4 ml/min (liquid volume). The nitrogen carrier gas was mixed with 1.4 l/min helium prior to injection of the liquid silicon source. The lines to the reactor were heated to 110ºC.

The temperature of the glass substrate during deposition and the deposition time are described in Table 6. TABLE 3 TABLE 4 TABLE 5 TABLE 6

The process of the invention enables silica coatings to be applied with good smoothness and uniformity at high rates, especially at temperatures below the swimming pool temperature, so that silica coatings can be conveniently applied to a ribbon on float glass in the annealing tunnel cooling channel or in the cooling tunnel gap (facilitating the application of a silica coating over a previous coating applied in the swimming pool). Little or no undesirable pre-reaction is observed and powder formation is not a problem. The gases used can be premixed prior to introduction into the coating chamber adjacent to the glass, enabling a laminar flow application process to be used; this obviously enables high conversion efficiency. In addition, alkoxysilanes, for example TEOS, can be used; such compounds have good chemical stability, are easy to handle and are readily available at reasonable costs.

Claims (23)

1. A method for applying a silicon oxide coating to hot glass at a temperature below 600°C, in which the hot glass is brought into contact with a gaseous mixture of a silicon source and ozone-enriched oxygen, characterized in that the molar ratio of ozone to silicon source in the gaseous mixture is in the range from 0.01:1 to 0.4:1. 2. The method of claim 1, wherein the hot glass in the form of a hot glass ribbon is brought into contact with the gaseous mixture during the floating glass manufacturing process downstream of the swimming pool. 3. A method according to claim 1 or claim 2, wherein the hot glass is at a temperature in the range of 200°C to 600°C. 4. A method according to claim 1 or claim 2, wherein the hot glass is at a temperature in the range of 200ºC to 575ºC. 5. A method according to claim 1 or claim 2, wherein the hot glass is at a temperature in the range of 225°C to 500°C. 6. A method according to claim 1 or claim 2, wherein the hot glass is at a temperature in the range of 375ºC to 525ºC. 7. A method according to any one of the preceding claims, wherein the hot glass is brought into contact with the gaseous mixture substantially at atmospheric pressure. 8. A process according to any one of the preceding claims, wherein a silane is used as silicon source. 9. The method of claim 8, wherein SiH4 is used as silicon source. 10. The method of claim 8, wherein an alkylsilane is used as silicon source. 11. The method of claim 10, wherein dimethylsilane is used as silicon source. 12. The method of claim 10, wherein trimethylsilane is used as silicon source. 13. Process according to one of the preceding claims, wherein an alkoxysilane is used as silicon source. 14. The method of claim 13, wherein tetraethoxysilane is used as silicon source. 15. Process according to claims 1 to 7, wherein a siloxane is used as silicon source. 16. The method of claim 15, wherein a cyclic siloxane is used as silicon source. 17. The method of claim 10, wherein octamethylcyclotetrasiloxane is used as silicon source. 18. Process according to claims 1 to 17, wherein a silicon halide is used as silicon source. 19. The method of claim 18, wherein silicon tetrachloride is used as the silicon source. 20. Process according to claims 1 to 7, wherein the silicon source contains a mixture of two or more silicon compounds. 21. A process according to any one of the preceding claims, wherein the oxygen is enriched with 0.5 to 5 wt.% ozone. 22. A method according to any one of the preceding claims, wherein the silicon oxide coating has a refractive index below 1.5. 23. Glass coated with a silicon oxide coating by a process of the preceding claims.